Earth penetrating tools include the rotatable cutter-type earth boring drill bit, such as a rolling cone rock bit. Rolling cone earth boring bits have a bit body with an upper end adapted for connection to a drill string and typically three bit legs which extend downward from the body. Depending from the lower portion of the bit body are a plurality of support arms, typically three in number. A bearing shaft extends inward and downward from each bit leg. A conventional rock bit bearing shaft is cylindrical and rotatably receives a cutter cone. The cutter cone is generally mounted on each bearing shaft and supported rotatably on bearings acting between the spindle and the inside of a spindle-receiving cavity in each cutter cone. The cutter cones have teeth or compacts on their exteriors for disintegrating earth formations as the cones rotate on the bearing shafts. One or more fluid nozzles are often formed on the underside of the bit body. The nozzles are typically positioned to direct drilling fluid passing downwardly from the drill string toward the bottom of the borehole being drilled. Drilling fluid washes away material removed from the bottom of the borehole and cleanses the cutter cones, carrying the cuttings and other debris radially outward and then upward within an annulus defined between the drill bit and the wall of the borehole.
There are several varieties of bearing systems used to support the cutter cones. These bearing systems typically consist of a combination of radial and thrust bearings that may be either sealed and lubricated, or unsealed and open to the drilling fluid, such as air. Contact wear surfaces for bearing shafts may consist of wear-resistant metals or non-metals such as tungsten carbide. In sealed bearing drill bits, seals which are placed across gaps between the cutter cones and respective bearing shafts to prevents debris from contaminating the bearing and also block the lubricant from leaking to the exterior. Various types of seals have been used, including elastomeric seals and metal-to-metal face seals. Open bearing drill bits operate without a seal, and often pass drilling fluids through the cutter bearings for cooling and lubrication. Open bearings often have ports to force drilling fluid through the bearing system to lubricate and cool bearing wear surfaces. In some instances air may be used for the drilling fluid and driven through the bearing to cool and to lubricate the bearings.
When operated in a borehole filled with liquid, hydrostatic pressure acts on the drill bit as a result of the weight of the column of drilling fluid. Temperature increases in the lubricant from heat transfer as the bit is lowered into the well and due to friction heat while rotating causes expansion of the lubricant. A sealed, grease-lubricated bearing drill bit contains a lubricant reservoir in the bit body that supplies lubricant to the bearing shafts. Each bearing shaft has a pressure compensation system that is mounted in the lubricant reservoirs in the bit body. Sealed bearing drill bits commonly use lubrication systems that include a lubricant pressure compensator to limit the pressure differential between the lubricant and the pressure in the borehole. A typical lubricant compensator includes a flexible diaphragm or a spring biased piston separating a lubricant reservoir and the lubricant from the borehole fluid. The diaphragm or spring biased piston moves in response to the pressure differential across it tending to equalize the pressure differential between the lubricant reservoir pressure and the borehole fluid pressure. A lubricant flow passage extends from the reservoir of the compensator to an exterior portion of the bearing shaft. The pressure compensation system has a communication port that communicates with the hydrostatic pressure on the exterior to equalize the pressure on the exterior with lubricant pressure in the passages and clearances within the drill bit. The viscous lubricant creates hydrodynamic lift as the cone rotates on the bearing shaft so that the load is partially supported by lubricant fluid film and partially by surface asperity to surface asperity contact.
Sealed bearing drill bit failures typically occur due to cutter bearing seals wearing until damaged and then the bearings fail before the cutting structure wears out. It is desired to extend the life of sealed bearing drill bits beyond the life of the seals.